The present invention relates to thin film transistors, and more particularly to the formation of silicon based thin film transistors on inexpensive, low-temperature plastic substrates. The present invention relates to a method of fabricating thin film transistors wherein heat sensitive substrates, such as inexpensive and flexible plastic substrates, may be used in place of standard glass, quartz, and silicon wafer-based substrates. A reflective coating for radiation protection of the plastic substrates is utilized during processing. The reflective coating layer is deposited above the plastic substrate to protect it from high intensity irradiation during processing by a laser or other high intensity radiation source.
Traditional techniques used in manufacturing high-performance polycrystalline silicon (poly-si) thin film transistors require processing temperatures of at least 600° C. This minimum temperature requirement is imposed by silicon crystallization and dopant activation anneals. Processes have recently been developed for crystallizing and doping amorphous silicon on a low cost, so-called low-temperature plastic substrates using a short pulsed high energy source in a selected environment, without heat propagation and build-up in the substrate so as to enable use of plastic substrates incapable of withstanding sustained processing temperatures higher than about 120° C. Such processes are described and claimed in U.S. Pat. No. 5,346,850 issued Sep. 13, 1994, to J. L. Kaschmitter et al. and U.S. Pat. No. 5,817,550 issued Oct. 6, 1998, to P. G. Carey et al., both assigned to the Assignee of the instant application.
As exemplified by the above-referenced U.S. Pat. No. 5,346,850, high performance polycrystalline silicon devices have been produced at low temperatures (<250° C.). This is accomplished by crystallizing the amorphous silicon layer (and activating dopants) with a short-pulse ultra-violet laser, such as an ArF excimer laser having a wavelength of 308 nm. The extremely short pulse duration (20–50 ns) allows the silicon thin film to melt and recrystallize without damaging the substrate or other layers in the device. Polycrystalline layers produced in this manner provide high carrier mobilities and enhanced dopant concentrations, resulting in better performance.
The present invention extends the capability of the above-mentioned method and processes for fabricating amorphous and polycrystalline channel silicon thin film transistors at temperatures sufficiently low to prevent damage to low cost, so-called low-temperature plastic substrates. The present invention utilizes a reflective coating for radiation protection of the plastic substrates during processing. A reflective coating layer is deposited above the plastic substrate to protect it from high intensity irradiation during processing by a laser or other high intensity radiation source. The process for fabrication of silicon thin film transistors on low-temperature plastic substrates, the thin film transistor, and the set of thin film transistor substrates for use in manufacturing thin film transistors of the present have different characteristics than existing thin film transistors. They have many and varied uses. For example, plastic displays and microelectronic circuits on flexible, rugged plastic substrates constructed in accordance with the present invention may be used for portable consumer electronics (video cameras, personal digital assistants, cell phones, etc.) or on large-area flat panel displays. Large area plastic displays are in need for high resolution large area flight simulators. Flexible detector arrays have use in radiation (X-ray, gamma-ray) detection. Silicon-on-insulator films may be used in radiation-hardened IC circuits. Many other uses exist and the development of the invention will produce additional uses.